DE 36 19 406 A1 discloses an adjusting device for adjustable rotor blades, in particular of a turbine or a propeller pump. With reference to the drawings of DE 36 19 406 A1, the known adjusting device includes a machine shaft 2 and an actuating shaft 6, via which the position or pitch of the rotor blades, which are rotatably disposed in a hub, is adjustable using a hydraulically-actuated piston 40. The actuating shaft 6 is rotatable with the machine shaft 2, but is axially displaceable relative to the machine shaft 2. A bearing 14 supports the actuating shaft 6 so that it is rotatable relative to the hydraulically-actuated piston 40. An actuating cylinder 42 accommodates the axially-displaceable piston 40 and is disposed on a machine housing in a stationary manner.